In recent years, GaN-based compound semiconductor materials, that is a nitride-based semiconductor, have become of interest as a semiconductor material for a light-emitting device that emits light of short wavelength. The GaN-based compound semiconductor is formed on a substrate of a sapphire single crystal, various oxides or a Group III-V compound through a metal-organic chemical vapor deposition method (MOCVD method), a molecular-beam epitaxy method (MBE method) or the like.
A GaN-based compound semiconductor material has a characteristic such as less diffusion of a current in a transverse direction. Therefore, the current is injected only into a semiconductor directly under an electrode, and light emitted in a light-emitting layer is not extracted to the outside because of being blocked off by the electrode. Therefore, in such a light-emitting device, a translucent positive electrode is usually used and light is extracted through the translucent positive electrode.
A conventional translucent positive electrode has a layer structure in which oxide of Ni, Co or the like is used in combination with a contact metal such as Au. There has recently been employed, as a positive electrode, a layer structure having translucency enhanced by thinning a film thickness of the contact metal as small as possible using an oxide having higher conductivity such as ITO, and thus light from a light-emitting layer can be efficiently extracted to the outside.
External quantum efficiency is used as an indicator for improving an output of such a light-emitting device. It can be said that a light-emitting device with high external quantum efficiency has a high output.
The external quantum efficiency is represented as a product obtained by multiplying internal quantum efficiency by light extraction efficiency.
The internal quantum efficiency is the percentage of energy that is converted into light among energy of a current injected into a device. On the other hand, the light extraction efficiency is the percentage of light that can be extracted to the outside among light generated in the inside of a semiconductor crystal.
It is considered that above-mentioned internal quantum efficiency of the light-emitting device is improved up to about 70 to 80% by an improvement of a crystal state and investigation of a structure, and a sufficient effect is obtained in the amount of injection current.
However, in not only a GaN-based compound semiconductor but also a light emitting diode (LED), light extraction efficiency to the injection current is generally low and it is difficult to say that internal emission to injection current is sufficiently extracted to the outside.
The reason why the light extraction efficiency is low is that reflection and absorption are repeated in a crystal and light cannot be extracted to the outside since a light-emitting layer in a GaN-based compound semiconductor has a very high refractive index of about 2.5 as compared air having a refractive index of 1 and a critical angle is small as about 25°.
In order to improve the light extraction efficiency of a light-emitting device, there have been proposed those in which light extraction efficiency is improved by roughening a light extraction surface and providing various angles on the light extraction surface.
For example, Patent Literature 1 discloses a gallium nitride-based compound semiconductor light-emitting device in which a surface of a top layer of a gallium nitride-based compound semiconductor is converted into a nonspecular surface through etching or the like.
Patent Literature 2 discloses a nitride semiconductor light-emitting device in which concavo-convex is formed on a surface of ITO.
However, in the light-emitting devices described in Patent Literatures 1 or 2, there was a problem that a driving voltage extremely increases due to etching damage when surface (light extraction surface) or the like of a top layer of the semiconductor is roughened through etching.
Patent Literature 3 discloses a light-emitting device in which a surface of a p-type GaN layer is processed into a concavo-convex surface through etching and, furthermore, metal such as Mg is added in a high concentration on the entire surface of the concavo-convex surface.
In Patent Literature 3, for example, the p-type GaN layer is etched using, as a mask, a resist layer having a stripe shape formed by pattern processing using a known photolithography method or the like to form a roughened surface made of a concavo-convex surface, and then an Mg layer is laminated on the concavo-convex surface and also the Mg layer is annealed, thereby diffusing Mg and further adding Mg to a surface side of the p-type GaN layer.
However, regarding the light-emitting device disclosed in Patent Literature 3, Mg is added to the entire surface of the p-type GaN layer. Therefore, in case a voltage is applied to a transparent electrode on the p-type GaN layer through a bonding pad, a current is not diffused on the entire surface of the transparent electrode and the current is concentrated on a semiconductor layer directly under the bonding pad, and thus luminous efficiency of the light-emitting device could not sometimes be improved.